Investment information - Fair value option (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	$ 2,048,103	$ 1,954,895
Investments, fair value option - gross unrealized gains	23,639	36,601
Investments, fair value option - gross unrealized losses	(339,657)	(93,405)
Investments, fair value option	1,732,085	1,898,091
Equities, fair value through net income - cost	88,134	78,031
Equities, fair value through net income - gross unrealized gains	1,855	2,360
Equities, fair value through net income - gross unrealized losses	(26,820)	(15,053)
Equities, fair value through net income	63,169	65,338
Term loans
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	1,113,510	1,113,212
Investments, fair value option - gross unrealized gains	986	7,340
Investments, fair value option - gross unrealized losses	(207,497)	(58,618)
Investments, fair value option	906,999	1,061,934
Fixed maturities \| Corporate bonds
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	296,918	221,024
Investments, fair value option - gross unrealized gains	4,038	8,430
Investments, fair value option - gross unrealized losses	(59,878)	(15,100)
Investments, fair value option	241,078	214,354
Fixed maturities \| U.S. government and government agency bonds
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	586	1,963
Investments, fair value option - gross unrealized gains	11	1
Investments, fair value option - gross unrealized losses	0	(2)
Investments, fair value option	597	1,962
Fixed maturities \| Asset-backed securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	198,262	200,361
Investments, fair value option - gross unrealized gains	2,231	3,329
Investments, fair value option - gross unrealized losses	(59,880)	(12,953)
Investments, fair value option	140,613	190,737
Fixed maturities \| Mortgage-backed securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	7,289	7,399
Investments, fair value option - gross unrealized gains	1,649	712
Investments, fair value option - gross unrealized losses	(409)	(405)
Investments, fair value option	8,529	7,706
Fixed maturities \| Non-U.S. government and government agency bonds
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	1,443	1,449
Investments, fair value option - gross unrealized gains	42	18
Investments, fair value option - gross unrealized losses	(103)	(11)
Investments, fair value option	1,382	1,456
Fixed maturities \| Municipal government and government agency bonds
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	252	380
Investments, fair value option - gross unrealized gains	1	0
Investments, fair value option - gross unrealized losses	0	(1)
Investments, fair value option	253	379
Short-term investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	348,059	325,542
Investments, fair value option - gross unrealized gains	542	3,817
Investments, fair value option - gross unrealized losses	(4,740)	(56)
Investments, fair value option	343,861	329,303
Other investments measured at net asset value
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	28,673	28,672
Investments, fair value option - gross unrealized gains	2,009	2,264
Investments, fair value option - gross unrealized losses	0	(475)
Investments, fair value option	30,682	30,461
Equities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, fair value option - cost or amortized cost	53,111	54,893
Investments, fair value option - gross unrealized gains	12,130	10,690
Investments, fair value option - gross unrealized losses	(7,150)	(5,784)
Investments, fair value option	$ 58,091	$ 59,799

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Accumulated Unrealized Gains Fair Value Option

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Equity FV-NI Gross Accumulated Losses

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Certain fixed maturity securities, equity securities, term loans, short-term investments and other investments held at fair value under the fair value option afforded by accounting guidance regarding the fair value option for financial assets and liabilities.

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